[Arran]

It could be the grid cap lead, some of those become flaky right where they solder onto the clip, and those can be effected by heat or vibration. I was just watching Don's Radio Shed and he was working on an Australian Briton radio (rebadged Phillips) and it had a down under version of a Micamold paper cap, it did more then cause noise though, it decided, periodically, to "transition" from being a paper bypass cap into a resistor. When I spotted it under the chassis whilst he was recapping I though "I hope Don checks that one out too, it looks like a Micamold" well it wasn't a Micamold, but it was a Micamold clone.
Real mica caps seldom cause trouble, well most of them don't, except for a certain series of post war Aerovox brand ones, that are all peanut butter brown instead of dark brown, I'm not sure why they fail but a lot of post war/Korean war era sets have them. What is strange is that I have also had them fail in very low voltage circuits, like coupling the loop antenna to the mixer section of the tuning condenser so the dielectric is clearly deteriorating due to age.
Regards
Arran